Wide-band amplifiers can reduce system costs and provide improved performance in many applications. Envelope tracking techniques can provide linear performance of wide-band amplifiers by running them deep into saturation. The improvement in efficiency is substantial with, for example, a 10%-20% improvement with LTE signals.
The problem for wide-band, envelope tracking amplifiers is that driving transistors deep into saturation produces substantial harmonics that can interfere with many receiving systems. While filter circuits can be used to reduce such harmonics, this results in reduced bandwidth and requires many discrete surface mount devices (SMDS). The additional filter circuits add power loss to the system and reduce the power amplifier efficiencies. A low pass, or band pass, filter can be used at the output of the amplifier to roll off the harmonics to acceptable levels. The hardest harmonic to filter is the second harmonic since it is the closest in frequency to the fundamental frequency. Typical power amplifier specifications require the second harmonic content to be 30-40 dbc.